Bulk transportation of chemical goods, plastics, grains, fertilizers and the like is practiced on a large scale now as a part of streamlining physical distribution ways and means, and a large plastic container is used for one of the transportation means. For example, a sack in the form of a rectangular parallelepiped of 6 to 12 m in length made of a cylindrical plastic film prepared by blown-film extrusion is actually used for transporting products stored therein.
Taking an example of this form by FIG. 1 showing a perspective view of a container, a container 1 is, as a whole, made of a sack body in the form of a rectangular parallelepiped prepared by cutting a cylindrical inflation film into the required length, folding both ends of the film longitudinally and bonding the superposed portion of the both ends. This large container 1 is used for bulk transportation of, e.g., solid terephthalic acid. Terephthalic acid is introduced into the container through a feeding end 3 by applying air pressure from outside. Terephthalic acid is taken out from a discharging end 4 to the outside. The facilitate filling and discharge the container has a ventilating end 5 as an air passage.
Up to this time, this ventilating end has been mounted on the container by making a short line slit through a sack body as for a body of a container, inserting a cylindrical film for a ventilating end into the slit and bonding both of them by heat-sealing.
Now, a process for mounting a ventilating end on a sack body will be explained in more in detail with reference to FIGS. 10 and 11. FIG. 10 is a perspective view showing the state of a ventilating end 100 mounted on a sack body 102, and FIG. 11 shows its cross-sectional view. At first, as shown in FIG. 10, a sack body 102 is prepared by folding both ends of a film 101 and bonding the superposed portion 103 of the folded ends by heat-sealing. At this time, by inserting a cylindrical film 104 into the superposed portion 103 and heat-sealing along the superposed portion 103, production of the sack body and mounting of the ventilating end can be attained in one process. During this time, as shown in FIG. 11, by inserting a fluorocarbon resin sheet 105 inside the cylindrical film 104, bonding both insides of the cylindrical film 104 to each other during heat-sealing can be avoided.
In this production process, a cone summit, or an acute summit is structurally formed in both corners of the heat-sealed portion 106 of the sack body 102 and the cylindrical film 104. On condition that the summit of the both corners is cone-shaped, stress due to air pressure in storing or discharging fillings concentrates on this spot to induce the generation of pinholes which start from the summit spot and extend along the heat-sealed portion 106. To this end, it has been hitherto taken such measures as to prevent lowering of pressure produced by pinholes, easy which are easily developed in the vicinity of the heat-sealed portion 106 and bursting of the container with locally applied large stress, by applying a pressure-sensitive tape 107 for reinforcing closely to (in the vicinity of) the heat-sealed portion 106.
However, these measures require much time taken in operation to result in a raise of cost, and to enhance the adhesive strength of the heat-sealed portion still more is also needed.